Assembly process

ABSTRACT

A method and apparatus for component to substrate assembly permits in situ reflow of a flip chip (or other suitable component) in a manner which promotes proper settling of the component as solder begins to flow at the contact points between the component and the substrate. The pick-up head of a placement machine heats the component while applying up to several grams of downward force that serves to level the component. The downward force (downforce) is accurately measured using an electronic force sensor such as a strain gauge, force sensitive resistor, or any other suitable type of force sensor. The initiation of solder reflow can be detected with the pick-up head by sensing a decrease in the downforce. At this instant, the downforce applied to the component with the pick-up head is decreased preferably to zero and the vacuum or other retention mechanism holding the component is then released, freeing the component from the pick-up head and permitting the component to properly self-center using the liquid solder&#39;s surface tension. Further, at the instant that solder reflow is detected, the pick-up head may optionally be displaced a short distance from the component. However, because the pick-up head must (where it is used to supply heat) continue to supply heat to complete the reflow of the solder, it is only displaced a minimal distance from the component so that heating by radiation continues to reflow the solder while the pick-up head is displaced from the chip. The approach is applicable to other assembly processes where downforce is helpful to stabilize a component prior to final bonding and a change in measured downforce indicated the beginning of melting, curing or another process which indicates that downforce can be removed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. no.09/801,559, filed on Mar. 7, 2001, now U.S. Pat. No. 6,605,500 entitled“Assembly Process” in the name of the same inventors and commonly ownedherewith.

RELATED APPLICATIONS

This application claims the benefit of provisional U.S. patentapplication Ser. No. 60/188,635 filed on Mar. 10, 2000 in the names ofEdison T. Hudson and Ernest H. Fischer and comnonly assigned herewith.

FIELD OF THE INVENTION

The present invention is related to the alignment and registration ofcomponents onto substrates in a machine placement environment. Moreparticularly, the present invention is directed a process for in situreflow of a flip chip-type semiconductor product in a manner whichpromotes proper settling of the chip onto a substrate as solder beginsto flow. The present invention may also be used with any type ofcomponent to substrate soldering or bonding where it is useful to applya downforce to assist in holding alignment prior to a change takingplace in the bonding material, the change affecting the applieddownforce.

BACKGROUND OF THE INVENTION

Robotic assembly equipment is well known in the art. Such equipmentincludes, for example, pick and place (or placement) machines. Aplacement machine is a robotic instrument for picking up electronic andsimilar parts from component feeders and placing them at their assignedlocations on a substrate such as a printed circuit board (PCB). Once allparts are placed, the PCB is placed in a reflow oven and solder pastedisposed on the PCB melts or “reflows” forming permanent electricalconnections between conductive pads on the PCB and electrical contacts,leads or “pins” on the electrical components.

Occasionally there are problems with the permanent electricalconnections. For example, two pads of the PCB may become inadvertentlybridged by solder, forming a short; the component may be mis-located;the component may prove faulty; and the like. In these situations, it isoften economically desirable to salvage the partially assembled PCBrather than to scrap it. In order to salvage the PCB, one must removethe faulty component, re-prepare the PCB surface, and place and solder anew component (or a cleaned component) in the correct position on thePCB. This process is termed “rework”. Reworiking thus involves reflowingthe solder of an identified target component (and not that of the entirePCB), removing the faulty component; cleaning and refluxing the PCB inthe location where the component is to be mounted, reinstalling thecomponent and reflowing the solder for the component.

In the past, most known placement systems locate the part over thesubstrate, place it, and then the part is released, placed in a reflowoven, and allowed to reflow. Generally the surface tension properties ofthe molten solder cause the pins of the part to more or less self-centeron corresponding pads of the substrate resulting in a good electricalcontact. Similarly, known rework systems rely on the self-centering onthe pins of the part to the pads of the substrate to achieve accurateplacement. While the existing systems operate relatively effectively, aspin densities increase, it is becoming more desirable to exertadditional control on the placement of flip chip-type parts,particularly as the value of such parts tends to be higher than otherelectronic parts used in the fabrication of PCBs.

BRIEF DESCRIPTION OF THE INVENTION

A method and apparatus for component to substrate assembly permits insitu reflow of a flip chip (or other suitable component) in a mannerwhich promotes proper settling of the component as solder begins to flowat the contact points between the component and the substrate. Thepick-up head of a placement machine heats the component while applyingup to several grams of downward force that serves to level thecomponent. The downward force (downforce) is accurately measured usingan electronic force sensor such as a strain gauge, force sensitiveresistor, or any other suitable type of force sensor. The initiation ofsolder reflow can be detected with the pick-up head by sensing adecrease in the downforce. At this instant, the downforce applied to thecomponent with the pick-up head is decreased preferably to zero and thevacuum or other retention mechanism holding the component is thenreleased, freeing the component from the pick-up head and permitting thecomponent to properly self-center using the liquid solder's surfacetension. Further, at the instant that solder reflow is detected, thepick-up head may optionally be displaced a short distance from thecomponent. However, because the pick-up head must (where it is used tosupply heat) continue to supply heat to complete the reflow of thesolder, it is only displaced a minimal distance from the component sothat heating by radiation continues to reflow the solder while thepick-up head is displaced from the chip. The approach is applicable toother assembly processes where downforce is helpful to stabilize acomponent prior to final bonding and a change in measured downforceindicated the beginning of melting, curing or another process whichindicates that downforce can be removed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent invention and, together with the detailed description, serve toexplain the principles and implementations of the invention.

In the drawings:

FIG. 1 is a schematic diagram of a portion of a placement machine inwhich an in situ flip chip assembly apparatus would operate inaccordance with a specific embodiment of the present invention.

FIG. 2A is an elevational cross-sectional diagram illustrating an insitu flip chip assembly apparatus in accordance with a specificembodiment of the present invention.

FIG. 2B is an enlargement of a portion of FIG. 2A illustrating thepick-up head portion of an in situ flip chip assembly apparatus inaccordance with a specific embodiment of the present invention.

FIGS. 3A and 3B are diagrams illustrating the construction of a heaterelement used in an in situ flip chip assembly apparatus in accordancewith a specific embodiment of the present invention.

FIG. 4 is a process flow diagram illustrating a process for in situ flipchip assembly in accordance with a specific embodiment of the presentinvention.

DETAILED DESCRIPTION

Embodiments of the present invention are described herein in the contextof an in-situ flip chip assembly process. Those of ordinary skill in theart will realize that the following detailed description of the presentinvention is illustrative only and is not intended to be in any waylimiting. Other embodiments of the present invention will readilysuggest themselves to such skilled persons having the benefit of thisdisclosure. For example, the present invention can be used in anycomponent to substrate bonding process where downforce is helpful toprovide stabilization of the component and a change, such as in abonding material like solder, an adhesive, a conductive adhesive, andthe like, can be detected with a force sensor so as to signify a time toremove the applied downforce. Such components may include flip chipsemiconductor packages, other types of semiconductor packages (e.g.,leaded, bumped, quad flat pack, tab, and the like) as well aselectro-optical components, electro-mechanical components,micro-electronic-machine (MEMS) devices, and the like. Reference willnow be made in detail to implementations of the present invention asillustrated in the accompanying drawings. The same reference indicatorswill be used throughout the drawings and the following detaileddescription to refer to the same or like parts.

In the interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application- and business-related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

In accordance with the present invention, certain components, processsteps, and/or data structures may be implemented using various types ofoperating systems, computing platforms, computer programs, and/orgeneral purpose machines. In addition, those of ordinary skill in theart will recognize that devices of a less general purpose nature, suchas hardwired devices, field programmable gate arrays (FPGAs),application specific integrated circuits (ASICs), or the like, may alsobe used without departing from the scope and spirit of the inventiveconcepts disclosed herein.

FIG. 1 is a schematic diagram of a portion of a placement machine inwhich an in situ flip chip assembly apparatus would operate inaccordance with a specific embodiment of the present invention. The insitu flip chip assembly apparatus of the present invention may becarried out with the apparatus illustrated in FIG. 1. A typicalplacement or rework machine 100 will include an imaging device 101 ofsome type to help guide a pick-up head 102 to accurately providerelative alignment between a target substrate 104 and a component 106being placed. Such imaging systems are well known to those of ordinaryskill in the art and any such suitable system may be used with thepresent invention. The imaging system may include computer 108 to carryout image processing and machine vision tasks as well as positioncontrol tasks for the placement/rework machine 100. Such systems arewell known in the prior art and will not be further described herein toavoid over-complicating the disclosure. While one camera is shown inFIG. 1, many such imaging systems use two cameras or a camera forsubstrate registration and another optical registration device forcomponent registration such as the LaserAlign product available fromCyberOptics Corporation of Golden Valleys Minn.

FIG. 2A is an elevational cross-sectional diagram illustrating an insitu flip chip assembly apparatus in accordance with a specificembodiment of the present invention. FIG. 2B is an enlargement of aportion of FIG. 2A illustrating the pick-up head portion of an in situflip chip assembly apparatus in accordance with a specific embodiment ofthe present invention. A vacuum line 200 in the pick-up head assembly202 creates a vacuum to hold the flip chip 204 prior to placement on thetarget substrate 104. When the pick-up head assembly 202 is properlylocated over the target substrate 104, the pick-up head 202 places theflip chip 204 onto the target substrate 104 and applies a downward (or“Z”) force. The downward force (or “downforce”) is variable andtypically begins in the range of about 2 to about 10 grams of force andmay be increased to a desired level and represents a compressive forcebetween the component (flip chip) and the substrate. Z-axis positioningis provided by the Z-axis drive screw 210 which is driven by the Z-axismotor 212 through Z-axis drive belt 214. The Z-axis motor 212 thuspositions the pick-up head assembly 202.vertically and applies desireddownforce. A Z-axis encoder 213 which may be a shaft encoder providesfeedback to computer 108 in a conventional manner.

A force sensor 206 is incorporated into the pick-up head 202 for sensingthe relative force being applied to the flip chip 204. The force sensor206 is preferably an electronic force sensor such as a strain gauge,force sensitive resistor (FSR), or any other suitable type of forcesensor and the sample rate on the sensor 206 is typically higher than1000 samples per second. This information is passed to computer 108.Those of ordinary skill in the art will now realize that any suitablesensor for measuring force can also be used.

A T-axis motor 216 positions the pick-up head rotationally through areduction gear 218. An encoder 220 which may be a shaft encoder providesT-axis positional feedback to computer 108.

The entire pick-up assembly 230 shown in FIG. 2A is mounted on an X-Ypositioning system 232 for X-Y positioning in a horizontal planeparallel to substrate 104.

FIGS. 3A and 3B are diagrams illustrating the construction of a heaterelement 208 (FIGS. 2A and 2B) used in an in situ flip chip assemblyapparatus in accordance with a specific embodiment of the presentinvention. Heater element 208 as illustrated in FIGS. 3A and 3B isincorporated into the pick-up head 202 and supplies heat sufficient toreflow the solder between the flip chip 204 and the target substrate104. Heater element 208 is preferably a ceramic heater formed of aceramic substrate 300. A resistive element 302 on the heater-side of theheater element 208 provides electrical heating and is coupled to a powersupply (not shown) through vias 304, 306 with electrical contacts 308,310. The heater element 208 is temperature controlled and includes anelectronic temperature sensor 312 such as a thermocouple or resistancetemperature detector device. This is coupled to conventional temperaturecontrol circuitry with electrical contacts 314, 316. An aperture 318 inheater element 208 allows the heater element 208 to be positioned abouta vacuum pick-up nozzle of pick-up head 202. The heater element 208produces both infra-red and thermal radiation which promotes radiantheating of the flip chip 204 as well as conductive heating throughcontact. It will also now be realized that any appropriate type ofheater may also be used for applying heat to the solder. For example, ahot air supply may be used as may laser or focused light heaters, allsuch systems being commercially available from a number of vendors andwell known to those of ordinary skill in the art. It is also possible toinitiate heating at a special workstation of the placement machine, asis presently done for some large ceramic packaged semiconductorproducts, and complete the heating with a pick-up head mounted heater.

In operation, the pick-up head 202 of the in-situ flip chip assemblyapparatus picks up a flip chip from a component feeder or othercomponent storage unit and places it down solder bumps facing down inthe proper location on a target substrate 104. A downward force isapplied to the flip chip 204 by the pick-up head 202, and heat isapplied to both the flip chip 204 and target substrate 104 by contactthrough the pick-up head 202 and radiant heat from the pick-up head 202.While the heat and force are applied, the force sensor 206 is used todetermine the exact instant that the solder between the flip chip 204and target substrate 104 begins to melt. This happens at the moment theforce sensor 206 senses a decrease in the upward force being returnedfrom the flip chip 204. At the moment the force sensor 206 senses themelting point of the solder, the pick-up head 202 begins reducing thedownward force being applied to the flip chip 204 to zero and releasesthe vacuum which holds the flip chip 204. When the vacuum holding theflip chip 204 is released, the pick-up head 202 backs off from flip chip204 a slight distance. This distance is very small however, typicallyabout 0.5 millimeters, and permits the continued radiant heating of theflip chip 204 and target substrate 104 while the flip chip 204 isdetached from the pick-up head 202. The flip chip 204 then properlysettles in place onto the target substrate 104 using the liquid tensionof the melting solder.

In addition to placing flip chips onto bare target substrates, the insitu flip chip assembly apparatus of the present invention can alsoplace flip chips onto pre-assemble circuit boards. These circuit boardsmay already have been mass reflowed and tested prior to having the flipchip applied in situ by the in situ flip chip assembly apparatus.

FIG. 4 is a process flow diagram illustrating a process for in situ flipchip assembly in accordance with a specific embodiment of the presentinvention. Turning now to FIG. 4, process 400 begins at block 402 wherethe pick-up head with a component gripped in position is positioned inthe X, Y and T directions for placement on a substrate. At block 404 thepick-up head is lowered in the Z direction until a desired downforce ismeasured by a force detector (element 206 in FIG. 2A). At block 406 theheating element is powered to begin the reflow process in the vicinityof the component. At block 408 the downforce is monitored. Sincedownforce will decrease when the solder begins to reflow, the change indownforce can be used as a trigger to reduce all downforce pressure andoptionally to remove the pick-up head a small distance so that allheating of the component/substrate is due to radiative heating. At block410, control passes back to block 408 until a reduction in downforcepressure is detected. Once the reduction is detected, control passes toblock 412 and all contact force is reduced by reducing the pressureapplied by the Z-axis drive screw 210. At block 414, optionally thepick-up head is moved a small distance away (e.g., approximately 0.5 mm)from the component to continue heating by radiation only. Once thereflow process is completed, the heating element is depowered at block416 and the pick-up head is removed at block 418.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art having thebenefit of this disclosure that many more modifications than mentionedabove are possible without departing from the inventive concepts herein.The invention, therefore, is not to be restricted except in the spiritof the appended claims.

What is claimed is:
 1. An apparatus for in situ assembly of a flip chipsemiconductor package to a substrate, comprising: means for positioninga pick-up head holding the package over the substrate; means forlowering the package into contact with the substrate; means for applyinga downforce to the package; means for applying heat to the package;means for monitoring the downforce; and means for removing all downforcewhen a reduction in downforce is detected.
 2. An apparatus in accordancewith claim 1, further comprising: means for displacing the pick-up heada small distance from the package in response to said reduction indownforce.
 3. An apparatus in accordance with claim 2, furthercomprising: means for continuing to apply heat for a time afterdisplacing the pick-up head a small distance.
 4. A program storagedevice readable by a machine, tangibly embodying a program ofinstructions executable by the machine to perform a method for in situassembly of a flip chip semiconductor package to a substrate, the methodincluding: positioning a pick-up head holding the package over thesubstrate; lowering the package into contact with the substrate;applying a downforce to the package; applying heat to the package;monitoring the downforce; and removing all downforce when a reduction indownforce is detected.
 5. A device in accordance with claim 4, themethod further comprising: displacing the pick-up head a small distancefrom the package in response to said reduction in downforce.
 6. A devicein accordance with claim 5, the method further comprising: continuing toapply heat for a time after said displacing.
 7. An apparatus forassembly of a semiconductor package to a substrate, comprising: aplacement machine operating under the control of a computer; a pick-uphead positioned in X, Y and T directions by the placement machine, thepick-up head for picking up a package and placing it onto a targetsubstrate and applying a downforce thereto; a retention mechanismincorporated into the pick-up head to hold the package to the pick-uphead; a heater incorporated the said pick-up head for supplying heatcapable of reflowing solder disposed between the package and the targetsubstrate; and a force sensor incorporated into the pick-up head forsensing the downforce being applied to the package, wherein theapparatus is responsive to a reduction in the downforce brought about bythe initiation of reflow of the solder to completely remove alldownforce applied by the pick-up head.
 8. An apparatus in accordancewith claim 7, wherein said heater is temperature controlled and includesan electronic temperature sensor.
 9. An apparatus in accordance withclaim 8, wherein said electronic temperature sensor is a thermocouple.10. An apparatus in accordance with claim 8, wherein said electronictemperature sensor is a resistance-based temperature sensor.
 11. Anapparatus in accordance with claim 7, wherein said force sensor is astrain gauge.
 12. An apparatus in accordance with claim 7, wherein saidforce sensor is a force sensitive resistor.
 13. An apparatus forassembling a semiconductor package to a substrate, comprising: means forholding the package on a pick-up head of a placement machine; means forplacing the package onto the substrate; means for applying a downforceto the package; means for heating the package and the substrate from aheater disposed on the pick-up head; means for sensing the downforcebeing applied to the package; and means for decreasing the applieddownward force in response to sensing a decrease in the downforce. 14.An apparatus in accordance with claim 13, further comprising: means forreleasing the package from the pick-up head responsive to said means fordecreasing.
 15. An apparatus in accordance with claim 14, furthercomprising: means for displacing the pick-up head a small distance fromthe package responsive to said means for releasing.
 16. An apparatus inaccordance with claim 15, further comprising: means for continuing saidheating for a time after said pick-up head is displaced.
 17. A programstorage device readable by a machine, tangibly embodying a program ofinstructions executable by the machine to perform a method forassembling a semiconductor package to a substrate, the method including:holding the package on a pick-up head of a placement machine; placingthe package onto the substrate; applying a downforce to the package;heating the package and the substrate from a heater disposed on thepick-up head; sensing the downforce being applied to the package; anddecreasing the applied downward force in response to sensing a decreasein the downforce.
 18. A device in accordance with claim 17, the methodfurther comprising: releasing the package from the pick-up head inresponse to said decreasing.
 19. A device in accordance with claim 18,the method further comprising: displacing the pick-up head a smalldistance from the package after said releasing.
 20. A device inaccordance with claim 19, the method further comprising: continuing saidheating for a time after said displacing.
 21. An apparatus for assemblyof a component to a substrate, comprising: means for positioning apick-up head holding the component over the substrate; means forlowering the component into contact with the substrate; means forapplying a compressive force between the component and the substrate;means for performing a bonding process to bond the component to thesubstrate; means for monitoring the compressive force; and means forremoving all compressive force when a reduction in compressive force isdetected.
 22. An apparatus in accordance with claim 21, wherein saidmeans for performing includes means for applying heat to the component.23. An apparatus in accordance with claim 22, further comprising: meansfor displacing the pick-up head a small distance from the component inresponse to a reduction in compressive force.
 24. An apparatus inaccordance with claim 23, further comprising: means for continuing toapply heat for a time after displacing the pick-up head.
 25. A programstorage device readable by a machine, tangibly embodying a program ofinstructions executable by the machine to perform a method for assemblyof a component to a substrate, the method including: positioning apick-up head holding the component over the substrate; lowering thecomponent into contact with the substrate; applying a compressive forcebetween the component and the substrate; performing a bonding process tobond the component to the substrate; monitoring the compressive force;and removing all compressive force when a reduction in compressive forceis detected.
 26. A device in accordance with claim 25, wherein saidperforming includes applying heat to the component.
 27. A device inaccordance with claim 26, said method further comprising: displacing thepick-up head a small distance from the component in response to saidreduction in compressive force.
 28. A device in accordance with claim27, said method further comprising: continuing to apply heat for a timeafter said displacing.
 29. An apparatus for assembly of a component to asubstrate, comprising: a placement machine operating under the controlof a computer; a pick-up head positioned in X, Y and T directions by theplacement machine, the pick-up head for picking up a component andplacing it onto a target substrate and applying a downforce thereto; aretention mechanism incorporated into the pick-up head to hold thecomponent to the pick-up head; a force sensor incorporated into thepick-up head for sensing the downforce being applied to the component,where the apparatus is responsive to a reduction in the downforce tocompletely remove all downforce applied by the pick-up head.
 30. Anapparatus in accordance with claim 29, further comprising: a heaterincorporated the said pick-up head for supplying heat to the component.